In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically, comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. If, after imaging, the imaged regions of the imageable layer are removed in the developing process revealing the underlying hydrophilic surface of the substrate, the precursor is positive-working. Conversely, if the unimaged regions are removed by the developing process, the precursor is negative-working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque regions of the mask. The mask is usually a photographic negative of the desired image. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, the mask may change slightly in dimension due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging of imageable elements, which obviates the need for imaging through a negative, is becoming increasingly important in the printing industry. Negative-working, alkaline developable imageable elements that comprise compounds that form an acid on thermal imaging have been developed for use with infrared lasers. However, dot gain and dot stability is a problem in these systems. Dot gain occurs when the size of a printed dot is larger than the specified size. Dot stability measures the variation in dot size with variation in exposure. In addition, these systems are developed in an aqueous alkaline developer that has a pH of 12 or more. Because of their high pH, disposal of these developers without creating environmental problems can be difficult. In addition, because the developer absorbs carbon dioxide from the air, its activity can change during use.
Thus, a need exists for negative-working imageable elements that can be imaged without exposure through a negative but do not have these disadvantages.